(a) Field of the Invention
The present invention relates to a multi-component composite film, and a method for the same, and more particularly, to a multi-component composite film used for a polymer electrolyte for electrochemical devices, a method for the same, and a polymer electrolyte system applied the same.
(b) Description of the Related Art
Energy storage technology has drawn attention and has been extensively studied recently, as cellular phones, camcorders, portable computers, and electric motors are commonly use energy storage devices. In particular, a secondary battery that is capable of charging and discharging has drawn the most attention, and new electrodes and polymer electrolytes have been developed to enhance its characteristics, such as cycle life and capacity. The polymer electrolyte is important to fabrication of a stable and high-quality battery, and development of the desired polymer electrolyte is needed.
Electrolytes are classified as liquid electrolyte and solid electrolyte. The liquid electrolyte comprises a salt dissolved and dissociated in an organic solvent, and it has high ionic conductivity. The liquid electrolyte is generally used together with a polymer separator, e.g. a polymer film such as a polyolefin forming pores that has ionic conductivity by filling the liquid electrolyte in the pores. The ionic conductivity varies depending on the porosity of the polymer separator, and the polyolefin separator generally has the ionic conductivity of about 1 mS/cm. But the liquid electrolyte may leak out of the polymer separator due to its high fluidity. In addition, the liquid electrolyte is not adhesive to the polymer separator, and there is an interface between them. The polymer separator has high mechanical strength due to its high crystallinity, and it is neither over-swelled nor decomposed.
The solid electrolyte has an ionic conductivity insufficient to be used in a battery. In order to improve the ionic conductivity of the solid electrolyte, a gellable polymer electrolyte has been suggested, in which liquid electrolyte comprising a salt dissolved in an organic solvent is impregnated in a solid polymer electrolyte, e.g. a hybrid-type electrolyte as disclosed in U.S. Pat. No. 5,418,091, prepared by Bellcore Co. However, when the gellable polymer electrolyte is used for an electrolyte of a battery, there are problems in battery-assembly due to its low mechanical strength, and the polymer electrolyte may be over-swelled, its thickness may increase, and energy density may decrease due to a decrease in the density of the polymer electrolyte, even though the polymer electrolyte has a thickness greater than 50 μm in order to insulate between electrodes and obtain a sufficient mechanical strength in a battery. Furthermore, a plasticizer having a low molecular weight that is harmful to the environment is used, an extraction process thereof is needed, and its mass production is difficult.
The polymer electrolyte requires electrochemical stability in working voltage, and thermal and chemical stability. Preferably, an ionic conductivity of the polymer electrolyte is greater than 1 mS/cm at room temperature, its wet-out rate is better than that of non-aqueous electrolyte, and it has high chemical-resistance. In addition, it is preferable that the polymer electrolyte adhesion is sufficient to decrease the interfacial resistance between the electrolyte and electrodes during a battery-assembly, and that it has enough mechanical strength during the battery-assembly. However, it is known that when the ionic conductivity increases, the mechanical strength of the polymer electrolyte deteriorates, and vice versa.
To increase both the ionic conductivity and the mechanical strength, it is disclosed that a porous polymer layer and a gellable multi-layer film are used for a separator in U.S. Pat. Nos. 5,639,573, 5,716,421, 5,631,103, and 5849443, and in European Patent Application No. 0933824A2. The porous polymer layer comprises a material that is hard to swell due to restrictive absorption of liquid electrolyte, and the exemplary material includes polyethylene, polypropylene, polytetrafluoroethylene, polyethylene terephthalate, polybutyleneterephthalate, and polyethylenenaphthalate, and a multi-layer film or bled film thereof. The gellable polymer comprises a self-gellable and self-swellable material when it meets liquid electrolyte, and the exemplary material includes polyvinylidenefluoride, polyurethane, polyethyleneoxide, polyacrylonitrile, polymethylmethacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidinone, and polytetraethylene glycol diacrylate, and a copolymer thereof.
When the aforementioned separator is used, the mechanical properties can be improved, however the polymer electrolyte has ionic conductivity lower than those of the porous polymer and the liquid electrolyte dissolved thereof, resulting from the ionic conductivity-resistance of the gellable polymer. U.S. Pat. Nos. 5,631,103, and 5,849,433 disclose that a plasticizer having a low molecular weight, such as dibutyl phthalate, is used in order to enhance the ionic conductivity of the separator. However, the plasticizer is harmful to the environment, and it makes mass production of a battery difficult. In addition, a multi-layer film prepared by the aforementioned method has a dense gellable polymer layer having no pores, its ionic conductivity-resistance increases undesirably, and an interfacial adhesion strength between the porous polymer layer and the gellable polymer layer weakens.